Chapter I: On Water and Its Compounds (1)

Water is found almost everywhere in nature, and in all three physical states. As vapour, water occurs in the atmosphere, and in this form it is distributed over the entire surface of the earth. The vapour of water in condensing, by cooling, forms snow, rain, hail, dew, and fog. One cubic metre (or 1,000,000 cubic centimetres, or 1,000 litres, or 35·316 cubic feet) of air can contain at 0° only 4·8 grams of water, at 20° about 17·0 grams, at 40° about 50·7 grams; but ordinary air only contains about 60 per cent. of this maximum. Air containing less than 40 per cent. is felt to be dry, whilst air which contains more than 80 per cent. of the same maximum is considered as distinctly damp.[1] Water in the liquid state, in falling as rain and snow, soaks into the soil and collects together into springs, lakes, rivers, seas, and oceans. It is absorbed from the soil by the roots of plants, which, when fresh, contain from 40 to 80 per cent. of water by weight. Animals contain about the same amount of water. In a solid state, water appears as snow, ice, or in an intermediate form between these two, which is seen on mountains covered with perpetual snow. The water of rivers,[2] springs, oceans and seas, lakes, and wells contains various substances in solution mostly salt,--that is, substances resembling common table salt in their physical properties and chief chemical transformations. Further, the quantity and nature of these salts differ in different waters.[3] Everybody knows that there are salt, fresh, iron, and other waters. The presence of about 3-1/2 per cent. of salts renders sea-water[4] bitter to the taste and increases its specific gravity. Fresh water also contains salts, but only in a comparatively small quantity. Their presence may be easily proved by simply evaporating water in a vessel. On evaporation the water passes away as vapour, whilst the salts are left behind. This is why a crust (incrustation), consisting of salts, previously in solution, is deposited on the insides of kettles or boilers, and other vessels in which water is boiled. Running water (rivers, &c.) is charged with salts, owing to its being formed from the collection of rain water percolating through the soil. While percolating, the water dissolves certain parts of the soil. Thus water which filters or passes through saline or calcareous soils becomes charged with salts or contains calcium carbonate (chalk). Rain water and snow are much purer than river or spring water. Nevertheless, in passing through the atmosphere, rain and snow succeed in catching the dust held in it, and dissolve air, which is found in every water. The dissolved gases of the atmosphere are partly disengaged, as bubbles from water on heating, and water after long boiling is quite freed from them.

[1] In practice, the chemist has to continually deal with gases, and
gases are often collected over water; in which case a certain
amount of water passes into vapour, and this vapour mixes with
the gases. It is therefore most important that he should be able
to calculate the amount of water or of _moisture in air and other
gases_. Let us imagine a cylinder standing in a mercury bath, and
filled with a dry gas whose volume equals _v_, temperature _t_°,
and pressure or tension _h_ mm. (_h_ millimetres of the column
of mercury at 0°). We will introduce water into the cylinder in
such a quantity that a small part remains in the liquid state, and
consequently that the gas will be saturated with aqueous vapour;
the volume of the gas will then increase (if a larger quantity
of water be taken some of the gas will he dissolved in it, and
the volume may therefore he diminished). We will further suppose
that, after the addition of the water, the temperature remains
constant; then since the volume increases, the mercury in the
cylinder falls, and therefore the pressure as well as the volume
is increased. In order to investigate the phenomenon we will
artificially increase the pressure, and reduce the volume to the
original volume _v_. Then the pressure or tension will be greater
than _h_, namely _h_ + _f_, which means that by the introduction
of aqueous vapour the pressure of the gas is increased. The
researches of Dalton, Gay-Lussac, and Regnault showed that this
increase is equal to the maximum pressure which is proper to
the aqueous vapour at the temperature at which the observation
is made. The maximum pressure for all temperatures may be found
in the tables made from observations on the pressure of aqueous
vapour. The quantity _f_ will be equal to this maximum pressure
of aqueous vapour. This may be expressed thus: the maximum
tension of aqueous vapour (and of all other vapours) saturating
a space in a vacuum or in any gas is the same. This rule is
known as _Dalton's law_. Thus we have a volume of dry gas _v_,
under a pressure _h_, and a volume of moist gas, saturated with
vapour, under a pressure _h_ + _f_. The volume _v_ of the dry gas
under a pressure _h_ + _f_ occupies, from Boyle's law, a volume
_vh_/_h_ + _f_; consequently the volume occupied by the aqueous
vapour under the pressure _h_ + _f_ equals _v_-_vh_/(_h_ + _f_),
or _vf_/(_h_ + _f_). Thus the volumes of the dry gas and of the
moisture which occurs in it, at a pressure _h_ + _f_, are in the
ratio _f_ : _h_. And, therefore, if the aqueous vapour saturates
a space at a pressure _n_, the volumes of the dry air and of the
moisture which is contained in it are in the ratio (_n_-_f_) :
_f_, where _f_ is the pressure of the vapour according to the
tables of vapour tension. Thus, if a volume N of a gas saturated
with moisture be measured at a pressure H, then the volume of
the gas, when dry, will be equal to N[(H-f)/H]. In fact, the
entire volume N must be to the volume of dry gas _x_ as H is to
H-_f_; therefore, N : _x_ = H : H-_f_, from which _x_ = N[(H-f)/H].
Under any other pressure--for instance, 760 mm.--The volume of
dry gas will be _x_H/760, or (H-_f_)/760, and we thus obtain
the following practical rule: If a volume of a gas saturated
with aqueous vapour be measured at a pressure H mm., then the
volume of dry gas contained in it will be obtained by finding the
volume corresponding to the pressure H, less the pressure due
to the aqueous vapour at the temperature observed. For example,
37·5 cubic centimetres of air saturated with aqueous vapour were
measured at a temperature of 15·3°, and under a pressure of 747·3
mm. of mercury (at 0°). What will be the volume of dry gas at 0°
and 760 mm.?

The pressure of aqueous vapour corresponding to 15·3° is equal to
12·9 mm., and therefore the volume of dry gas at 15·3° and 747·3
mm. is equal to 37·5 × (747·3-12·9)/747·3; at 760 mm. it will be
equal to 37·5 × (734·4/760); and at 0° the volume of dry gas will
be 37·5 × (734·4/760) × 273/(273 + 15·3) = 34·31 c.c.

From this rule may also be calculated what fraction of a volume
of gas is occupied by moisture under the ordinary pressure at
different temperatures; for instance, at 30° C. _f_ = 31·5,
consequently 100 volumes of a moist gas or air, at 760 mm.,
contain a volume of aqueous vapour 100 × (31·5/760), or 4·110; it
is also found that at 0° there is contained 0·61 p.c. by volume,
at 10° 1·21 p.c., at 20° 2·29 p.c., and at 50° up to 12·11 p.c.
From this it may be judged how great an error might be made in
the measurement of gases by volume if the moisture were not taken
into consideration. From this it is also evident how great are
the variations in volume of the atmosphere when it loses or gains
aqueous vapour, which again explains a number of atmospheric
phenomena (winds, variation of pressure, rainfalls, storms, &c.)

If a gas is not saturated, then it is indispensable that the
degree of moisture should be known in order to determine the
volume of dry gas from the volume of moist gas. The preceding
ratio gives the maximum quantity of water which can be held in
a gas, and the degree of moisture shows what fraction of this
maximum quantity occurs in a given case, when the vapour does
not saturate the space occupied by the gas. Consequently, if the
degree of moisture equals 50 p.c.--that is, half the maximum--then
the volume of dry gas at 760 mm. is equal to the volume of dry
gas at 760 mm. multiplied by (_h_-0·5_f_)/760, or, in general, by
(_h_-_rf_)/760 where _r_ is the degree of moisture. Thus, if it
is required to measure the volume of a moist gas, it must either
be thoroughly dried or quite saturated with moisture, or else
the degree of moisture determined. The first and last methods
are inconvenient, and therefore recourse is usually had to the
second. For this purpose water is introduced into the cylinder
holding the gas to be measured; it is left for a certain time so
that the gas may become saturated, the precaution being taken
that a portion of the water remains in a liquid state; then the
volume of the moist gas is determined, from which that of the
dry gas may be calculated. In order to find the _weight of the
aqueous vapour_ in a gas it is necessary to know the weight of a
cubic measure at 0° and 760 mm. Knowing that one cubic centimetre
of air in these circumstances weighs 0·001293 gram, and that
the density of aqueous vapour is 0·62, we find that one cubic
centimetre of aqueous vapour at 0° and 760 mm. weighs 0·0008 gram,
and at a temperature _t_° and pressure _h_ the weight of one cubic
centimetre will be 0·0008 × _h_/760 × 273/(273 + _t_). We already
know that _v_ volumes of a gas at a temperature _t_° pressure _h_
contain _v_ × _f_/_h_ volumes of aqueous vapour which saturate it,
therefore the weight of the aqueous vapour held in _v_ volumes of
a gas will be

_v_ x 0·0008 × _f_/760 × 273/(273 + _t_).

Accordingly, the weight of water which is contained in one volume
of a gas depends only on the temperature and not on the pressure.
This also signifies that evaporation proceeds to the same extent
in air as in a vacuum, or, in general terms (this is _Dalton's
law_), vapours and gases diffuse into each other as if into a
vacuum. In a given space, at a given temperature, a constant
quantity of vapour enters, whatever be the pressure of the gas
filling that space.

From this it is clear that if the weight of the vapour contained
in a given volume of a gas be known, it is easy to determine the
degree of moisture _r_ = _p_/(_v_ × 0·0008) × 760/_t_ × (273 +
_t_)/273. On the is founded the very exact determination of the
degree of moisture of air by the weight of water contained in a
given volume. It is easy to calculate from the preceding formula
the number of grams of water contained at any pressure in one
cubic metre or million cubic centimetres of air saturated with
vapour at various temperatures; for instance, at 30° _f_ = 31·5,
hence _p_ = 29·84 grams.

The laws of Mariotte, Dalton, and Gay-Lussac, which are here
applied to gases and vapours, are not entirely exact, but are
approximately true. If they were quite exact, a mixture of several
liquids, having a certain vapour pressure, would give vapours of
a very high pressure, which is not the case. In fact the pressure
of aqueous vapour is slightly less in a gas than in a vacuum,
and the weight of aqueous vapour held in a gas is slightly less
than it should be according to Dalton's law, as was shown by the
experiments of Regnault and others. This means that the tension
of the vapour is less in air than in a vacuum. The difference
does not, however, exceed 5 per cent. of the total pressure of
the vapours. This _decrement in vapour tension_ which occurs in
the intermixture of vapours and gases, although small, indicates
that there is then already, so to speak, a beginning of chemical
change. The essence of the matter is that in this case there
occurs, as on contact (see preceding footnote), an alteration in
the motions of the atoms in the molecules, and therefore also a
change in the motion of the molecules themselves.

In the uniform intermixture of air and other gases with aqueous
vapour, and in the capacity of water to pass into vapour and
form a uniform mixture with air, we may perceive an instance of
a physical phenomenon which is analogous to chemical phenomena,
forming indeed a transition from one class of phenomena to
the other. Between water and dry air there exists a kind of
affinity which obliges the water to saturate the air. But such a
homogeneous mixture is formed (almost) independently of the nature
of the gas in which evaporation takes place; even in a vacuum
the phenomenon occurs in exactly the same way as in a gas, and
therefore it is not the property of the gas, nor its relation to
water, but the property of the water itself, which compels it to
evaporate, and therefore in this case chemical affinity is not yet
operative--at least its action is not clearly pronounced. That it
does, however, play a certain part is seen from the deviation from
Dalton's law.

[2] In falling through the atmosphere, water dissolves the gases of
the atmosphere, nitric acid, ammonia, organic compounds, salts
of sodium, magnesium, and calcium, and mechanically washes
out a mixture of dust and microbes which are suspended in the
atmosphere. The amount of these and certain other constituents
is very variable. Even in the beginning and end of the same
rainfall a variation which is often very considerable may be
remarked. Thus, for example, Bunsen found that rain collected
at the beginning of a shower contained 3·7 grams of ammonia per
cubic metre, whilst that collected at the end of the same shower
contained only O·64 gram. The water of the entire shower contained
an average of 1·47 gram of ammonia per cubic metre. In the course
of a year rain supplies an acre of ground with as much as 5-1/2
kilos of nitrogen in a combined form. Marchand found in one cubic
metre of snow water 15·63, and in one cubic metre of rain water
10·07, grams of sodium sulphate. Angus Smith showed that after a
thirty hours' fall at Manchester the rain still contained 34·3
grams of salts per cubic metre. A considerable amount of organic
matter, namely 25 grams per cubic metre, has been found in rain
water. The total amount of solid matter in rain water reaches 50
grams per cubic metre. Rain water generally contains very little
carbonic acid, whilst river water contains a considerable quantity
of it. In considering the nourishment of plants it is necessary
to keep in view the substances which are carried into the soil by
rain.

_River water_, which is accumulated from springs and sources
fed by atmospheric water, contains from 50 to 1,600 parts by
weight of salts in 1,000,000 parts. The amount of solid matter,
per 1,000,000 parts by weight, contained in the chief rivers is
as follows:--the Don 124, the Loire 135, the St. Lawrence 170,
the Rhone 182, the Dnieper 187, the Danube from 117 to 234, the
Rhine from 158 to 317, the Seine from 190 to 432, the Thames at
London from 400 to 450, in its upper parts 387, and in its lower
parts up to 1,617, the Nile 1,580, the Jordan 1,052. The Neva is
characterised by the remarkably small amount of solid matter it
contains. From the investigations of Prof. G. K. Trapp, a cubic
metre of Neva water contains 32 grams of incombustible and 23
grams of organic matter, or altogether about 55 grams. This is one
of the purest waters which is known in rivers. The large amount
of impurities in river water, and especially of organic impurity
produced by pollution with putrid matter, makes the water of many
rivers unfit for use.

The chief part of the soluble substances in river water consists
of the calcium salts. 100 parts of the solid residues contain
the following amounts of calcium carbonate--from the water of
the Loire 53, from the Thames about 50, the Elbe 55, the Vistula
65, the Danube 65, the Rhine from 55 to 75, the Seine 75, the
Rhone from 82 to 94. The Neva contains 40 parts of calcium
carbonate per 100 parts of saline matter. The considerable amount
of calcium carbonate which river water contains is very easily
explained from the fact that water which contains carbonic acid
in solution easily dissolves calcium carbonate, which occurs all
over the earth. Besides calcium carbonate and sulphate, river
water contains magnesium, silica, chlorine, sodium, potassium,
aluminium, nitric acid, iron and manganese. The presence of salts
of phosphoric acid has not yet been determined with exactitude
for all rivers, but the presence of nitrates has been proved with
certainty in almost all kinds of well-investigated river water.
The quantity of calcium phosphate does not exceed 0·4 gram in the
water of the Dnieper, and the Don does not contain more than 5
grams. The water of the Seine contains about 15 grams of nitrates,
and that of the Rhone about 8 grams. The amount of ammonia is
much less; thus in the water of the Rhine about 0·5 gram in June,
and 0·2 gram in October; the water of the Seine contains the same
amount. This is less than in rain water. Notwithstanding this
insignificant quantity, the water of the Rhine alone, which is not
so very large a river, carries 16,245 kilograms of ammonia into
the ocean every day. The difference between the amount of ammonia
in rain and river water depends on the fact that the soil through
which the rain water passes is able to retain the ammonia. (Soil
can also absorb many other substances, such as phosphoric acid,
potassium salts, &c.)

The waters of springs, rivers, wells, and in general of those
localities from which it is taken for drinking purposes, may
be injurious to health if it contains much organic pollution,
the more so as in such water the lower organisms (bacteria) may
rapidly develop, and these organisms often serve as the carriers
or causes of infectious diseases. For instance, certain pathogenic
(disease-producing) bacteria are known to produce typhoid, the
Siberian plague, and cholera. Thanks to the work of Pasteur,
Metchnikoff, Koch, and many others, this province of research has
made considerable progress. It is possible to investigate the
number and properties of the germs in water. In bacteriological
researches a gelatinous medium in which the germs can develop and
multiply is prepared with gelatin and water, which has previously
been heated several times, at intervals, to 100° (it is thus
rendered sterile--that is to say, all the germs in it are killed).
The water to be investigated is added to this prepared medium in
a definite and small quantity (sometimes diluted with sterilised
water to facilitate the calculation of the number of germs), it
is protected from dust (which contains germs), and is left at
rest until whole families of lower organisms are developed from
each germ. These families (colonies) are visible to the naked eye
(as spots), they may be counted, and by examining them under the
microscope and observing the number of organisms they produce,
their significance may be determined. The majority of bacteria
are harmless, but there are decidedly pathogenic bacteria, whose
presence is one of the causes of malady and of the spread of
certain diseases. The number of bacteria in one cubic centimetre
of water sometimes attains the immense figures of hundreds of
thousands and millions. Certain well, spring, and river waters
contain very few bacteria, and are free from disease-producing
bacteria under ordinary circumstances. By boiling water, the
bacteria in it are killed, but the organic matter necessary for
their nourishment remains in the water. The best kinds of water
for drinking purposes do not contain more than 300 bacteria in a
cubic centimetre.

The amount of gases dissolved in river water is much more constant
than that of its solid constituents. One litre, or 1,000 c.c., of
water contains 40 to 55 c.c. of gas measured at normal temperature
and pressure. In winter the amount of gas is greater than in
summer or autumn. Assuming that a litre contains 50 c.c. of
gases, it may be admitted that these consist, on an average, of
20 vols. of nitrogen, 20 vols of carbonic anhydride (proceeding
in all likelihood from the soil and not from the atmosphere), and
of 10 vols. of oxygen. If the total amount of gases be less, the
constituent gases are still in about the same proportion; in many
cases, however, carbonic anhydride predominates. The water of many
deep and rapid rivers contains less carbonic anhydride, which
shows their rapid formation from atmospheric water, and that they
have not succeeded, during a long and slow course, in absorbing a
greater quantity of carbonic anhydride. Thus, for instance, the
water of the Rhine, near Strasburg, according to Deville, contains
8 c.c. of carbonic anhydride, 16 c.c. of nitrogen, and 7 c.c. of
oxygen per litre. From the researches of Prof. M. R. Kapoustin and
his pupils, it appears that in determining the quality of a water
for drinking purposes, it is most important to investigate the
composition of the dissolved gases, more especially oxygen.

[3] _Spring water_ is formed from rain water percolating through the
soil. Naturally a part of the rain water is evaporated directly
from the surface of the earth and from the vegetation on it. It
has been shown that out of 100 parts of water falling on the earth
only 36 parts flow to the ocean; the remaining 64 are evaporated,
or percolate far underground. After flowing underground along
some impervious strata, water comes out at the surface in many
places as springs, whose temperature is determined by the depth
from which the water has flowed. Springs penetrating to a great
depth may become considerably heated, and this is why hot mineral
springs, with a temperature of up to 30° and higher, are often met
with. When a spring water contains substances which endow it with
a peculiar taste, and especially if these substances are such as
are only found in minute quantities in river and other flowing
waters, then the spring water is termed a _mineral water_. Many
such waters are employed for medicinal purposes. Mineral waters
are classed according to their composition into--(_a_) saline
waters, which often contain a large amount of common salt; (_b_)
alkaline waters, which contain sodium carbonate; (_c_) bitter
waters, which contain magnesia; (_d_) chalybeate waters, which
hold iron carbonate in solution; (_e_) aërated waters, which are
rich in carbonic anhydride; (_f_) sulphuretted waters, which
contain hydrogen sulphide. Sulphuretted waters may be recognised
by their smell of rotten eggs, and by their giving a black
precipitate with lead salts, and also by their tarnishing silver
objects. Aërated waters, which contain an excess of carbonic
anhydride, effervesce in the air, have a sharp taste, and redden
litmus paper. Saline waters leave a large residue of soluble
solid matter on evaporation, and have a salt taste. Chalybeate
waters have an inky taste, and are coloured black by an infusion
of galls; on being exposed to the air they usually give a brown
precipitate. Generally, the character of mineral waters is mixed.
In the table below the analyses are given of certain mineral
springs which are valued for their medicinal properties. The
quantity of the substances is expressed in millionths by weight.

Column Headings:
A: Calcium salts
B: Sodium chloride
C: Sodium sulphate
D: Sodium carbonate
E: Potassium iodide and bromide

+-------+-------+--------+-------+-------+-----+------+
| | | | | | | |
| | [A] | [B] | [C] | [D] | [E] | [F] |
| | | | | | | |
+-------+-------+--------+-------+-------+-----+------+
| | | | | | | |
| I. | 1,928 | -- | 152 | -- | -- | 24 |
| II. | 816 | 386 | 1,239 | 26 | -- | 43 |
| III. | 1,085 | 1,430 | 1,105 | -- | 4 | 90 |
| IV. | 343 | 3,783 | 16 | 3,431 | -- | 14 |
| V. | 3,406 | 15,049 | -- | -- | 2 | -- |
| VI. | 352 | 3,145 | -- | 95 | 35 | 50 |
| VII. | 308 | 1,036 | 2,583 | 1,261 | 4 | 178 |
| VIII. | 1,726 | 9,480 | -- | -- | 40 | 120 |
| IX. | 551 | 2,040 | 1,150 | 999 | -- | 1 |
| X. | 285 | 558 | 279 | 3,813 | -- | -- |
| | | | | | | |
| XI. | 340 | 910 | Iron and aluminium {1,020 |
| | | | sulphates: {1,660 |
+-------+-------+--------+----------------------------+

Column Headings:
G: Iron carbonate
H: Magnesium salts
I: Silica
J: Carbonic anhydride
K: Sulphuretted hydrogen
L: Total solid contents

+-------+------+-------+-----+-------+-----+-----------+
| | | | | | | |
| | [G] | [H] | [I] | [J] | [K] | [L] |
+-------+------+-------+-----+-------+-----+-----------+
| | | | | | | |
| I. | -- | 448 | 152 | 1,300 | 80 | 2,609 |
| II. | 9 | 257 | 46 | 1,485 | -- | 2,812 |
| III. | -- | 187 | 65 | 1,326 | 11 | 3,950 |
| IV. | -- | 251 | 112 | 2,883 | -- | 7,950 |
| V. | 17 | 1,587 | 229 | -- | 76 | 20,290 |
| VI. | 1 | 260 | 11 | 20 | -- | 3,970 |
| VII. | 4 | 178 | 75 | -- | -- | 5,451 |
| VIII. | 26 | 208 | 40 | -- | -- | 11,790 |
| IX. | 30 | 209 | 50 | 2,740 | -- | 4,070 |
| X. | 7 | 45 | 45 | 2,268 | -- | 5,031 |
| | | | | {Sulphuric |
| XI. | | 940 | 190 | 2,550 {and hydrochloric |
| | | | | 330 {acids |
+-------+------+-------+-----+-------------------------+

I. Sergieffsky, a sulphur water, Gov. of Samara (temp. 8° C.),
analysis by Clause. II. Geléznovodskya water source No. 10,
near Patigorsk, Caucasus (temp. 22·5°), analysis by Fritzsche.
III. Aleksandroffsky, alkaline-sulphur source, Patigorsk (temp.
46·5°), average of analyses by Herman, Zinin and Fritzsche.
IV. Bougountouksky, alkaline source, No. 17, Essentoukah,
Caucasus (temp. 21·6°), analysis by Fritzsche. V. Saline water,
Staro-Russi, Gov. of Novgorod, analysis by Nelubin. VI. Water from
artesian well at the factory of state papers, St. Petersburg,
analysis by Struve. VII. Sprüdel, Carlsbad (temp. 83·7°), analysis
by Berzelius. VIII. Kreuznach spring (Elisenquelle), Prussia
(temp. 8·8°), analysis by Bauer. IX. Eau de Seltz, Nassau,
analysis by Henry. X. Vichy water, France, analysis by Berthier
and Puvy. XI. Paramo de Ruiz, New Granada, analysis by Levy; it is
distinguished by the amount of free acids.

[4] _Sea water_ contains more non-volatile saline constituents than
the usual kinds of fresh water. This is explained by the fact that
the waters flowing into the sea supply it with salts, and whilst a
large quantity of vapour is given off from the surface of the sea,
the salts remain behind. Even the specific gravity of sea water
differs considerably from that of pure water. It is generally
about 1·02, but in this and also in respect of the amount of salts
contained, samples of sea water from different localities and from
different depths offer rather remarkable variations. It will be
sufficient to point out that one cubic metre of water from the
undermentioned localities contains the following quantity in grams
of solid constituents:--Gulf of Venice, 19,122; Leghorn Harbour
24,312; Mediterranean, near Cetta, 37,665; the Atlantic Ocean
from 32,585 to 35,695,; the Pacific Ocean from 35,233 to 34,708.
In closed seas which do not communicate, or are in very distant
communication, with the ocean, the difference is often still
greater. Thus the Caspian Sea contains 6,300 grams; the Black
Sea and Baltic 17,700. Common salt forms the chief constituent
of the saline matter of sea or ocean water; thus in one cubic
metre of sea water there are 25,000-31,000 grams of common salt,
2,600-6,000 grams of magnesium chloride, 1,200-7,000 grams of
magnesium sulphate, 1,500-6,000 grams of calcium sulphate, and
10-700 grams of potassium chloride. The small amount of organic
matter and of the salts of phosphoric acid in sea water is very
remarkable. Sea water (the composition of which is partially
discussed in Chapter X.) contains, in addition to salts of common
occurrence, a certain and sometimes minute amount of the most
varied elements, even gold and silver, and as the mass of water of
the oceans is so enormous these 'traces' of rare substances amount
to large quantities, so that it may be hoped that in time methods
will be found to extract even gold from sea water, which by means
of the rivers forms a vast reservoir for the numerous products
of the changes taking place on the earth's surface. The works of
English, American, German, Russian, Swedish, and other navigators
and observers prove that a study of the composition of sea water
not only explains much in the history of the earth's life, but
also gives the possibility (especially since the researches of
C. O. Makaroff of the St. Petersburg Academy) of fixing one's
position in the ocean in the absence of other means, for instance,
in a fog, or in the dark.

In general terms water is called pure when it is clear and free from insoluble particles held in suspension and visible to the naked eye, from which it may be freed by filtration through charcoal, sand, or porous (natural or artificial) stones, and when it possesses a clean fresh taste. It depends on the absence of any taste, decomposing organic matter, on the quantity of air[5] and atmospheric gases in solution, and on the presence of mineral substances to the amount of about 300 grams per ton (or 1000 kilograms per cubic metre, or, what is the same, 300 milligrams to a kilogram or a litre of water), and of not more than 100 grams of organic matter.[6] Such water is suitable for drinking and every practical application, but evidently it is not pure in a chemical sense. A _chemically pure water_ is necessary not only for scientific purposes, as an independent substance having constant and definite properties, but also for many practical purposes--for instance, in photography and in the preparation of medicines--because many properties of substances in solution are changed by the impurities of natural waters. Water is usually purified by distillation, because the solid substances in solution are not transformed into vapours in this process. Such _distilled_ water is prepared by chemists and in laboratories by boiling water in closed metallic boilers or stills, and causing the steam produced to pass into a condenser--that is, through tubes (which should be made of tin, or, at all events, tinned, as water and its impurities do not act on tin) surrounded by cold water, and in which the steam, being cooled, condenses into water which is collected[7] in a receiver. By standing exposed to the atmosphere, however, the water in time absorbs air, and dust carried in the air. Nevertheless, in distillation, water retains, besides air, a certain quantity of volatile impurities (especially organic) and the walls of the distillation apparatus are partly corroded by the water, and a portion, although small, of their substance renders the water not entirely pure, and a residue is left on evaporation.[8]

[5] The taste of water is greatly dependent on the quantity of
dissolved gases it contains. These gases are given off on
boiling, and it is well known that, even when cooled, boiled
water has, until it has absorbed gaseous substances from the
atmosphere, quite a different taste from fresh water containing
a considerable amount of gas. The dissolved gases, especially
oxygen and carbonic anhydride, have an important influence on
the health. The following instance is very instructive in this
respect. The Grenelle artesian well at Paris, when first opened,
supplied a water which had an injurious effect on men and animals.
It appeared that this water did not contain oxygen, and was in
general very poor in gases. As soon as it was made to fall in
a cascade, by which it absorbed air, it proved quite fit for
consumption. In long sea voyages fresh water is sometimes not
taken at all, or only taken in a small quantity, because it spoils
by keeping, and becomes putrid from the organic matter it contains
undergoing decomposition. Fresh water may he obtained directly
from sea-water by distillation. The distilled water no longer
contains sea salts, and is therefore fit for consumption, but
it is very tasteless and has the properties of boiled water. In
order to render it palatable certain salts, which are usually held
in fresh water, are added to it, and it is made to flow in thin
streams exposed to the air in order that it may become saturated
with the component parts of the atmosphere--that is, absorb gases.

[6] _Hard water_ is such as contains much mineral matter, and
especially a large proportion of calcium salts. Such water,
owing to the amount of lime it contains, does not form a lather
with soap, prevents vegetables boiled in it from softening
properly, and forms a large amount of incrustation on vessels
in which it is boiled. When of a high degree of hardness, it
is injurious for drinking purposes, which is evident from the
fact that in several large cities the death-rate has been found
to decrease after introducing a soft water in the place of a
hard water. _Putrid water_ contains a considerable quantity of
decomposing organic matter, chiefly vegetable, but in populated
districts, especially in towns, chiefly animal remains. Such
water acquires an unpleasant smell and taste, by which stagnant
bog water and the water of certain wells in inhabited districts
are particularly characterised. Water of this kind is especially
injurious at a period of epidemic. It may be partially purified
by being passed through charcoal, which retains the putrid and
certain organic substances, and also certain mineral substances.
Turbid water may be purified to a certain extent by the addition
of alum, which aids, after standing some time, the formation of
a sediment. Condy's fluid (potassium permanganate) is another
means of purifying putrid water. A solution of this substance,
even if very dilute, is of a red colour; on adding it to a putrid
water, the permanganate oxidises and destroys the organic matter.
When added to water in such a quantity as to impart to it an
almost imperceptible rose colour it destroys much of the organic
substances it contains. It is especially salutary to add a small
quantity of Condy's fluid to impure water in times of epidemic.

The presence in water of one gram per litre, or 1,000 grams per
cubic metre, of any substance whatsoever, renders it unfit and
even injurious for consumption by animals, and this whether
organic or mineral matter predominates. The presence of 1 p.c. of
chlorides makes water quite salt, and produces thirst instead of
assuaging it. The presence of magnesium salts is most unpleasant;
they have a disagreeable bitter taste, and, in fact, impart to sea
water its peculiar taste. A large amount of nitrates is only found
in impure water, and is usually injurious, as they may indicate
the presence of decomposing organic matter.

[7]

Distilled water may be prepared, or distillation in general
carried on, either in a metal still with worm condenser (fig. 4)
or on a small scale in the laboratory in a glass retort (fig.
5) heated by a lamp. Fig. 5 illustrates the main parts of the
usual glass laboratory apparatus used for distillation. The steam
issuing from the retort (on the right-hand side) passes through a
glass tube surrounded by a larger tube, through which a stream of
cold water passes, by which the steam is condensed, and runs into
a receiver (on the left-hand side).

[8] One of Lavoisier's first memoirs (1770) referred to this question.
He investigated the formation of the earthy residue in the
distillation of water in order to prove whether it was possible,
as was affirmed, to convert water into earth, and he found that
the residue was produced by the action of water on the sides of
the vessel containing it, and not from the water itself. He proved
this to be the case by direct weighing.

For certain physical and chemical researches, however, it is necessary to have perfectly pure water. To obtain it, a solution of potassium permanganate is added to distilled water until the whole is a light rose colour. By this means the organic matter in the water is destroyed (converted into gases or non-volatile substances). An excess of potassium permanganate does no harm, because in the next distillation it is left behind in the distillation apparatus. The second distillation should take place in a platinum retort with a platinum receiver. Platinum is a metal which is not acted on either by air or water, and therefore nothing passes from it into the water. The water obtained in the receiver still contains air. It must then be boiled for a long time, and afterwards cooled in a vacuum under the receiver of an air pump. Pure water does not leave any residue on evaporation; does not in the least change, however long it be kept; does not decompose like water only once distilled or impure; and it does not give bubbles of gas on heating, nor does it change the colour of a solution of potassium permanganate.

Water, purified as above described, has constant _physical_ and _chemical properties_. For instance, it is of such water only that one cubic centimetre weighs one gram at 4° C.--_i.e._ it is only such pure water whose specific gravity equals 1 at 4° C.[9] Water in a solid state forms crystals of the hexagonal system[10] which are seen in snow, which generally consists of star-like clusters of several crystals, and also in the half-melted scattered ice floating on rivers in spring time. At this time of the year the ice splits up into spars or prisms, bounded by angles proper to substances crystallising in the hexagonal system.

[9] Taking the generally-accepted specific gravity of water at its
greatest density--_i.e._ at 4° as one--it has been shown by
experiment that the specific gravity of water at different
temperatures is as follows:

At 0° 0·99987 | At 30° 0·99574
" +10° 0·99974 | " 40° 0·99233
" 15° 0·99915 | " 50° 0·98817
" 20° 0·99827 | " 100° 0·95859

A comparison of all the data at present known shows that the
variation of the specific gravity S_{t} with the temperature
_t_ (determined by the mercurial thermometer) maybe expressed
(Mendeléeff 1891) by the formula

S_{t} = 1 - (_t_-4)^{2}/(94·1 + _t_) (703·51-_t_) 1·9

+-----------+-------------+---------------------------+-----------+
| t° C. | | Variation of sp. gr. with | |
| according |Sp. gr. S_{t}| a rise of | Volume |
| to the |(at 4° = +--------------+------------+taking vol.|
| mercurial | 1,000,000) |Temp. per 1°C.|Pressure per| at 4° = 1 |
|thermometer| | or ds/dt |1 atmosphere| |
| | | | or ds/dp | |
+-----------+-------------+--------------+------------+-----------+
| -10 | 998,281 | +264 | +54 | 1,001,722 |
| 0 | 999,873 | +65 | +50 | 1,000,127 |
| 10 | 999,738 | -85 | +47 | 1,000,262 |
| 20 | 998,272 | -203 | +45 | 1,001,731 |
| 30 | 995,743 | -299 | +43 | 1,004,276 |
| 50 | 988,174 | -450 | +40 | 1,011,967 |
| 70 | 977,948 | -569 | +39 | 1,022,549 |
| 90 | 965,537 | -670 | +41 | 1,035,692 |
| 100 | 958,595 | -718 | +42 | 1,043,194 |
| 120 | 943,814 | -810 | +43 | 1,060,093 |
| 160 | 907,263 | -995 | +55 | 1,102,216 |
| 200 | 863,473 | -1,200 | +73 | 1,158,114 |
+-----------+-------------+--------------+------------+-----------+

If the temperature be determined by the hydrogen thermometer,
whose indications between 0° and 100° are slightly lower than the
mercurial (for example, about 0·1° C. at 20°), then a slightly
smaller sp. gr. will be obtained for a given _t_. Thus Chappuis
(1892) obtained 0·998233 for 20°. Water at 4° is taken as the
basis for reducing measures of length to measures of weight and
volume. The _metric, decimal, system_ of measures of weights and
volumes is generally employed in science. The starting point of
this system is the metre (39·37 inches) divided into decimetres
(= 0·1 metre), centimetres (= 0·01 metre), millimetres (= 0·001
metre), and micrometres (= one millionth of a metre). A cubic
decimetre is called a _litre_, and is used for the measurement
of volumes. The weight of a litre of water at 4° in a vacuum is
called a kilogram. One thousandth part of a kilogram of water
weighs one _gram_. It is divided into decigrams, centigrams,
and milligrams (= 0·001 gram). An English pound equals 453·59
grams. The great advantage of this system is that it is a decimal
one, and that it is universally adopted in science and in most
international relations. _All the measures cited in this work are
metrical._ The units most often used in science are:--Of length,
the centimetre; of weight, the gram; of time, the second; of
temperature, the degree Celsius or Centigrade. According to the
most trustworthy determinations (Kupfer in Russia 1841, and Chaney
in England 1892), the weight of a c. dcm. of water at 4° in vacuo
is about 999·9 grms. For ordinary purposes the weight of a c.
dcg. may be taken as equal to a kg. Hence the litre (determined
by the weight of water it holds) is slightly greater than a cubic
decimetre.

[10] As solid substances appear in independent, regular, crystalline
forms which are dependent, judging from their cleavage or
lamination (in virtue of which mica breaks, up into laminae,
and Iceland spar, &c., into pieces bounded by faces inclined to
each other at angles which are definite for each substance), on
an inequality of attraction (cohesion, hardness) in different
directions which intersect at definite angles the determination
of crystalline form therefore affords one of the most important
characteristics for identifying definite chemical compounds. The
elements of crystallography which comprise a special science
should therefore he familiar to all who desire to work in
scientific chemistry. In this work we shall only have occasion
to speak of a few crystalline forms, some of which are shown in
figs. 6 to 12.

The temperatures at which water passes from one state to another are taken as fixed points on the thermometer scale; namely, the zero corresponds with the temperature of melting ice, and the temperature of the steam disengaged from water boiling at the normal barometric pressure (that is 760 millimetres measured at 0°, at the latitude of 45°, at the sea level) is taken as 100° of the Celsius scale. Thus, the fact that water liquefies at 0° and boils at 100° is taken as one of its properties as a definite chemical compound. The weight of a litre of water at 4° is 1,000 grams, at 0° it is 999·8 grams. The weight of a litre of ice at 0° is less--namely, 917 grams; the weight of the same cubic measure of water vapour at 760 mm. pressure and 100° is only 0·60 gram; the density of the vapour compared with air = 0·62, and compared with hydrogen = 9.

These data briefly characterise the physical properties of water as a separate substance. To this may be added that water is a mobile liquid, colourless, transparent, without taste or smell, &c. Its latent heat of vaporisation is 534 units, of liquefaction 79 units of heat.[11] The large amount of heat stored up in water vapour and also in liquid water (for its specific heat is greater than that of other liquids) renders it available in both forms for heating purposes. The chemical reactions which water undergoes, and by means of which it is formed, are so numerous, and so closely allied to the reactions of many other substances, that it is impossible to describe the majority of them at this early stage of chemical exposition. We shall become acquainted with many of them afterwards, but at present we shall only cite certain compounds formed by water. In order to see clearly the nature of the various kinds of compounds formed by water we will begin with the most feeble, which are determined by purely mechanical superficial properties of the reacting substances.[12]

[11] Of all known liquids, water exhibits the greatest _cohesion_ of
particles. Indeed, it ascends to a greater height in capillary
tubes than other liquids; for instance, two and a half times as
high as alcohol, nearly three times as high as ether, and to a
much greater height than oil of vitriol, &c. In a tube one mm. in
diameter, water at 0° ascends 15·3 mm., measuring from the height
of the liquid to two-thirds of the height of the meniscus, and at
100° it rises 12·5 mm. The cohesion varies very uniformly with
the temperature; thus at 50° the height of the capillary column
equals 13·9 mm.--that is, the mean between the columns at 0°
and 100°. This uniformity is not destroyed even at temperatures
near the freezing point, and hence it may be assumed that at
high temperatures cohesion will vary as uniformly as at ordinary
temperatures; that is, the difference between the columns at 0°
and 100° being 2·8 mm., the height of the column at 500° should
be 15·2-(5 × 2·8) = 1·2 mm.; or, in other words, at these high
temperatures the cohesion between the particles of water would he
almost _nil_. Only certain solutions (sal ammoniac and lithium
chloride), and these only with a great excess of water, rise
higher than pure water in capillary tubes. The great cohesion of
water doubtless determines many of both its physical and chemical
properties.

The quantity of heat required to raise the temperature of one
part by weight of water from 0° to 1°, _i.e._ by 1° C., is called
the _unit of heat_ or calorie; the _specific heat of liquid
water_ at 0° is taken as equal to unity. The variation of this
specific heat with a rise in temperature is inconsiderable in
comparison with the variation exhibited by the specific heats of
other liquids. According to Ettinger, the specific heat of water
at 20° = 1·016, at 50° = 1·039, and at 100° = 1·073. The specific
heat of water is greater than that of any other known liquid;
for example, the specific heat of alcohol at 0° is 0·55--_i.e._
the quantity of heat which raises 55 parts of water 1° raises
100 parts of alcohol 1°. The specific heat of oil of turpentine
at 0° is 0·41, of ether 0·53, of acetic acid 0·5274, of mercury
0·033. Hence water is the best condenser or absorber of heat.
This property of water has an important significance in practice
and in nature. Water prevents rapid cooling or heating, and thus
tempers cold and heat. The specific heats of ice and aqueous
vapour are much less than that of water; namely, that of ice is
0·504, and of steam 0·48.

With an increase in pressure equal to one atmosphere, the
compressibility of water (_see_ Note 9) is 0·000047, of mercury
0·00000352, of ether 0·00012 at 0°, of alcohol at 13° 0·000095.
The addition of various substances to water generally decreases
both its compressibility and cohesion. The compressibility of
other liquids increases with a rise of temperature, but for water
it decreases up to 53° and then increases like other liquids.

The _expansion of water_ by heat (Note 9) also exhibits many
peculiarities which are not found in other liquids. The expansion
of water at low temperatures is very small compared with other
liquids; at 4° it is almost zero, and at 100° it is equal to
0·0008; below 4° it is negative--_i.e._ water on cooling then
expands, and does not decrease in volume. In passing into a solid
state, the specific gravity of water decreases; at 0° one c.c.
of water weighs 0·999887 gram, and one c.c. of ice at the same
temperature weighs only 0·9175 gram. The ice formed, however,
contracts on cooling like the majority of other substances. Thus
100 volumes of ice are produced from 92 volumes of water--that
is, water expands considerably on freezing, which fact determines
a number of natural phenomena. The freezing point of water falls
with an increase in pressure (0·007° per atmosphere), because in
freezing water expands (Thomson), whilst with substances which
contract in solidifying the melting point rises with an increase
in pressure; thus, paraffin under one atmosphere melts at 46°,
and under 100 atmospheres at 49°.

When liquid water passes into vapour, the cohesion of its
particles must be destroyed, as the particles are removed to
such a distance from each other that their mutual attraction
no longer exhibits any influence. As the cohesion of aqueous
particles varies at different temperatures, the quantity of heat
which is expended in overcoming this cohesion--or the _latent
heat of evaporation_--will for this reason alone be different at
different temperatures. The quantity of heat which is consumed in
the transformation of one part by weight of water, at different
temperatures, into vapour was determined by Regnault with great
accuracy. His researches showed that one part by weight of water
at 0°, in passing into vapour having a temperature _t_°, consumes
606·5 + 0·305_t_ units of heat, at 50° 621·7, at 100° 637·0, at
150° 652·2, and at 200° 667·5. But this quantity includes also
the quantity of heat required for heating the water from 0° to
_t_°--_i.e._ besides the latent heat of evaporation, also that
heat which is used in heating the water in a liquid state to a
temperature _t_°. On deducting this amount of heat, we obtain the
latent heat of evaporation of water as 606·5 at 0°, 571 at 50°,
534 at 100°, 494 at 150°, and only 453 at 200°, which shows that
the conversion of water at different temperatures into vapour
at a constant temperature requires very different quantities of
heat. This is chiefly dependent on the difference of the cohesion
of water at different temperatures; the cohesion is greater at
low than at high temperatures, and therefore at low temperatures
a greater quantity of heat is required to overcome the cohesion.
On comparing these quantities of heat, it will be observed that
they decrease rather uniformly, namely their difference between
0° and 100° is 72, and between 100° and 200° is 81 units of
heat. From this we may conclude that this variation will be
approximately the same for high temperatures also, and therefore
that no heat would be required for the conversion of water into
vapour at a temperature of about 400°. At this temperature, water
passes into vapour whatever be the pressure (see Chap. II. The
absolute boiling point of water, according to Dewar, is 370°,
the critical pressure 196 atmospheres). It must here be remarked
that water, in presenting a greater cohesion, requires a larger
quantity of heat for its conversion into vapour than other
liquids. Thus alcohol consumes 208, ether 90, turpentine 70,
units of heat in their conversion into vapour.

The whole amount of heat which is consumed in the conversion of
water into vapour is not used in overcoming the cohesion--that
is, in internal accomplished in the liquid. A part of this heat
is employed in moving the aqueous particles; in fact, aqueous
vapour at 100° occupies a volume 1,659 times greater than that
of water (at the ordinary pressure), consequently a portion of
the heat or work is employed in lifting the aqueous particles, in
overcoming pressure, or in external work, which may be usefully
employed, and which is so employed in steam engines. In order
to determine this work, let us consider the variation of the
maximum _pressure_ or _vapour tension of steam_ at different
temperatures. The observations of Regnault in this respect,
as on those preceding, deserve special attention from their
comprehensiveness and accuracy. The pressure or tension of
aqueous vapour at various temperatures is given in the adjoining
table, and is expressed in millimetres of the barometric column
reduced to 0°.